Fiducial and differential measurements of Higgs boson production cross sections at the LHC Sarah Heim, University of Pennsylvania Particle Physics Seminar University of Oxford, November 24 th 2015 References: ATLAS γγ: JHEP 1409 (2014) 112 ATLAS 4l: Phys. Lett. B738 (2014) 234-253 ATLAS combination: Phys. Rev. Lett. 115, 091801 (2015) CMS γγ: arXiv:1508.07819 (submitted to EPJC) CMS 4l: CMS-PAS-HIG-14-028 1
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Fiducial and differential measurements of Higgs boson … · 2015-11-24 · Properties of the Higgs boson 1. check of SM calculations, and MC generators used in the analyses feedback
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Fiducial and differential measurements of
Higgs boson production cross sections at the LHC
Sarah Heim, University of Pennsylvania
Particle Physics SeminarUniversity of Oxford, November 24th 2015
References:
ATLAS γγ: JHEP 1409 (2014) 112
ATLAS 4l: Phys. Lett. B738 (2014) 234-253
ATLAS combination: Phys. Rev. Lett. 115, 091801 (2015)
CMS γγ: arXiv:1508.07819 (submitted to EPJC)
CMS 4l: CMS-PAS-HIG-14-028
1
2The Higgs boson as a probe for new physics
The Standard Model of particle physics is not complete,
among other things it cannot explain many cosmological observations
Examples:
- gravity
- matter-antimatter asymmetry in the universe
- existence of dark matter
The discovery of the Higgs boson in 2012
opened a new sector of investigations
Examples:
- search for new particles interacting with the Higgs, like dark matter
- in the ggF production loop
- in association with the Higgs boson
- in Higgs boson decays
- search for additional Higgs bosons (SUSY predicts at least 5)
?
3Higgs boson production and decay
SM production modes (fractions at 8 TeV) Decay branching fractions
ggF (86.1%) VBF (6.9%)
VH (4.9%) ttH, bbH (2.2%)
/b
/b
125 GeV
ZZWW
4The Higgs boson – property measurements at the LHC
Measured properties- mass 125.09 GeV (125.4/125.0 for shown measurements)- spin/CP = 0+- couplings: agreement with SM predictions- differential cross sections
5Cross section definitions
σ i=N reco ,i
c i∗A i∗L∗B
Number of measured signal events
Integrated luminosity
Cross section
Correction for detector efficiency
Acceptance
bin i
σ fid ,i∗B=N reco ,i
ci∗L
Fiducial cross section=
cross section in fiducial volume(cuts applied to
generated events)
Fiducial volumes depend on event selection/decay channel
Branching fraction
6Why fiducial cross sections?
Correction for detector effects
- easy interpretation for theorists
- preserve measurements for posterity
No acceptance correction
- less model dependence
Compared to measurements of signal strength
- signal strength is a ratio:
observed signal events/expected signal events
- legacy from the search phase
- entangles experimental with theoretical uncertainties
- often split into categories with SM assumptions
7Why differential cross sections?
Properties of the Higgs boson
1. check of SM calculations,
and MC generators used in the analyses
feedback to theory groups
2. deviations from the Standard Model
predictions could be due to new physics
Example Higgs pT:
- modification of the spectrum due to production in association
with dark matter
- low pT sensitive to quark couplings
- high pT tail sensitive to modified couplings at high energies
8Pick variables
- fiducial cross section
- Higgs kinematics
- Higgs pT, rapidity
→ sensitive to production modes,
QCD modeling, PDF
- Jet properties
- njets, leading jet pT,
- VBF sensitive variables
→ sensitive to production modes,
QCD modeling
- Decay variables
- like |cosθ*|
(angle between Z/γ and incoming quark)
→ sensitive to spin/CP of Higgs boson
9
Part 1: H → 4l and H → γγ analyses
ATLAS & CMS
10The H → 4l channel
- 4 leptons in the final state, full reconstruction of Higgs kinematic variables
- small backgrounds due to requirement of 4 isolated leptons
- ZZ, WZ, Z+jets, top pairs (leptons from heavy flavor decays, faked by hadron jets)
- S/B around 125 GeV: 2-3
- statistically limited in run 1 due to small branching fraction:
ATLAS had 15.6 expected signal events after event selection for 8 TeV data
11Signal extraction in the H → 4l channel
In inclusive fiducial measurement and CMS differential:
- fit performed to the m4l distribution
In ATLAS differential measurement,
- background estimate subtracted
in a mass window around the Higgs peak (118-129 GeV)
12The H → γγ channel
- 2 photons in the final state, full reconstruction of Higgs kinematics
- sensitive to new particles in the decay loop
- more expected signal events, more background than H → 4l channel: